Method for making, by blow moulding, plastic hollow bodies, device and installation therefor

ABSTRACT

To blow plastic hollow bodies, each mold ( 7 ) has its respective compression chamber ( 21 ) connected to it, constituted by a cylinder ( 20 )-piston ( 19 ) assembly; pressure is initialized in the chamber; the fluid is compressed in the chamber; the chamber is connected ( 22, 24, 25 ) with the blank ( 9 ) when the piston position attains a predetermined position and chamber volume continues to be reduced to complete the blowing. The invention can be applied in particular to the blowing of small-volume hollow bodies.

The object of the invention is improvements to methods for manufacturinghollow bodies, namely containers such as bottles, flasks, etc., obtainedby blowing of plastic blanks in finishing molds; its object is also adevice and an installation for implementing the method.

It can be applied in particular but not exclusively to the manufactureof small capacity hollow bodies, typically on the order of a half literor less.

To manufacture a hollow body such as a container of a plastic such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN) orpolyvinyl chloride (PVC) in a single layer or multiple layers, it isknown to produce a plastic blank, then to place this blank while it isat a softening temperature in a finishing mold, and to inject a blowingfluid (typically air) in the blank to transform it into a recipient.

Thus, so-called extrusion-blowing methods are known in which the blanks,referred to as parisons, are extruded tubes. To produce a container, aparison is closed in a mold and air is injected into the parison.

Injection-blow molding methods are also known in which the blanks arepreforms obtained by injecting the plastic into the mold; then, afterthey are produced, the preforms are either transferred to the finishingmold and then immediately transformed into containers in the finishingmold (methods known as hot-cycle) or stored or transported before beingsoftened by heating and transformed into containers in the mold (methodsreferred to as cold cycle).

A disadvantage of known machines for manufacturing hollow bodies lies inthe generation and transfer of the blowing fluid to the blanks.

Indeed, blowing requires high pressure levels (typically ranging from 10to 20 bars in the case of extrusion-blowing and in the 40-bar range inthe case of injection-blow molding), thus causing considerableconsumption of fluid. Blowing a one-liter hollow body produced byextrusion-blowing requires 10 to 20 liters of fluid; blowing a one-literhollow body produced by injection-blow molding requires 40 liters offluid.

To obtain sufficient pressure and flow levels with respect to productionrates (1,200 containers per hour and per mold in the applicant'sinjection-blow molding machines), known machines are connected with atleast one compressor that supplies the high pressure necessary forblowing.

The compressor operates continuously to be able to supply the quantityof air required for all the molds comprising the installation. As forthe blowing, it takes place sequentially.

The primary disadvantages of such a structure are as follows:

first of all, due to the continuous operation of the compressor whereasblowing takes place sequentially, electrically or mechanicallycontrolled valves must be provided to allow or prohibit the insertion ofblowing fluid in the molds. These valves are continuously subjectedupstream to the high pressure levels required for blowing. Theytherefore must be resistant and have a costly, complex design. Due tothe operating speeds to which they are subjected, they deterioratequickly. They thus constitute consumables that should be replacedrelatively often:

the compressors used are accessory devices for the installations.Besides their cost and burdensomeness, it is necessary to provide fluidconnections with the installations. These connections increase the risksof malfunction (leaks in the event of disconnection);

in addition, the same compressor is generally used to supply severalinstallations. If this device fails, this may result in considerableproduction losses, since several installations may be dependent upon asingle compressor;

with installations using a rotating carrousel technology in which themolds are carried by the carrousel, it is necessary to provide arotating fluid connection to carry the high-pressure blowing fluid tothe molds. In this case as well, it is a matter of a highly sensitivepart with very restrictive manufacturing tolerances.

The objective of the invention is to remedy these disadvantages.

According to the invention, a method for manufacturing a hollow body byblowing a blank into a finishing mold with the help of a blowing fluid,is characterized in that it consists of connecting to each mold aseparate compression chamber formed by a cylinder-piston assembly;establishing an initial fluid pressure in the chamber when it is at itsmaximum volume; reducing the chamber volume to compress the fluid whilekeeping the chamber and the inside of the blank isolated; connecting thechamber and the blank when the chamber volume reaches a defined value inorder to initiate blowing of the blank by retention of the compressedfluid, and continuing to reduce chamber volume to a minimum whilemaintaining the connection with the blank and ending the blowing bycompressing the fluid volume remaining in the chamber and transferringto the blank.

By connecting to each mold a compression chamber inside which thepressure ranges from a minimum (initial pressure) to the blowingpressure, there is no longer a device continuously undergoing the latterpressure.

In a form of construction, a controlled valve is arranged in the circuitbetween the chamber and the mold, but this valve undergoes pressure riseand drop cycles upstream. It is thus less restrictively subjected tostress.

The invention reduces the length of the fluid connections since thechamber can be placed as closely as possible to the corresponding mold.

An installation is no longer dependent upon a compressor. As a result,if a chamber is defective, it is possible to keep the rest of theinstallation running at least provisionally. In addition, twoinstallations are not dependent on each other.

Lastly, high-pressure rotating fluid connections are no longer necessaryeven when the molds are carried by a carrousel, since each mold isconnected with its respective chamber.

Another advantage of the invention is that compression of the air causesits temperature to rise, which considerably promotes blowing when theblank and thus the hollow body is of a thermoplastic matter. Indeed, ifthe air temperature exceeds the material's softening temperature, itprevents the material from solidifying during blowing.

The blowing temperature depends in large part on the initial pressureand, of course, on the compression ratio. It also depends on thepredetermined piston position at which blowing is initiated.

This is why the initial pressure established in the compression chamberis preferably greater than the ambient pressure and, according toanother characteristic, is established at least in part by an externallow-pressure source.

Low-pressure source refers to an industrial source currently present incompanies, ranging from 1 to 15 bars and typically 7 bars, for example.

Because it is low, there are no connection problems like those with ahigh-pressure source, as used to occur with installations of the priorart. Leakage risks are limited and the technology of fixed as well asrotating connections is perfectly mastered and much simpler.

Thus, for example, with a compression chamber with an initial volume of1.5 liters, and initial pressure of 7 bars, an item of 300 cc isinflated at 35 bars (by initiating the connection between the chamberand the blank, thus the mold when the piston position is such thatchamber volume reaches 300 cc).

With the same chamber, only 5 bars would be attained if the initialpressure were ambient pressure. To attain 35 bars, and blow a 300-ccarticle starting with ambient pressure, the chamber would have to havean initial volume of 7×1.5 liters, i.e., 10.5 liters.

The installation for implementation would become somewhat cumbersome.

According to another characteristic, initial pressure is obtained atleast in part by returning the high-pressure fluid contained in thehollow body to the chamber when it is degassed.

According to another characteristic, the low-pressure circuit isconnected so as to only supply fluid when the blowing cycle isestablished. It brings all of the fluid only when no hollow body hasbeen blown, or when a blank has burst during blowing.

Indeed, after blowing, the total fluid circuit volume has increased by alevel corresponding to the difference between the final volume of thehollow body and the initial volume of the blank. Not counting theinitial volume of the blank and that of the connections between thechamber and the mold, and by taking the same parameters as previously,total fluid circuit volume would be 1,500 cc+300 cc after blowing, i.e.,1,800 cc filled with 10.5 liters of air, making it possible to obtain a5.8-bar residual—thus initial—pressure for the subsequent blowing in thechamber.

Thus, the external source should only bring the supplement in order toobtain the 7 bars necessary in the case in question.

In this way, one achieves savings on the order of 80% in terms ofexternal fluid brought in.

In this respect, the method is self-regulating: as indicated, if a blankaccidentally bursts during blowing, the external supply will be total atthe subsequent blowing.

According to another characteristic, a device for implementing themethod comprises a compression chamber connected to a mold andconsisting of a cylinder in which a piston is arranged; a fluid circuitconnecting the compression chamber with means to establish an initialpressure in the compression chamber; means to connect the compressionchamber and the inside of a blank placed in the mold when the pistonposition attains a predetermined position, and in that the length of thecylinder is such that after connecting, the piston's stroke continues tocomplete the blowing by transferring the fluid volume remaining in thechamber to the blank.

According to another characteristic, an installation comprising at leastone device for blowing a hollow body also comprises: a chassis as wellas a structure rotating around a pivot borne by the installation'schassis; at least one mold is attached to this rotating structure and isconnected to its respective device; a first tip of each cylinder-pistonassembly is connected to a respective first axle borne by the chassis,parallel to the pivot's axis and at a distance from it that defines thedesired piston stroke; a second tip of each assembly is connected to arespective second axle borne by the rotating structure toward aperipheral zone thereof.

Thus, due to the eccentricity between the first axle and the pivot, analternate movement of the piston is caused relative to the cylinder whenthe rotating structure rotates. In actual fact, the distance between thepivot and the first axle corresponds to half of the piston stroke.

In a preferred form of construction, the piston control stem isconnected to the first axle, and the cylinder is connected to the secondaxle.

According to another characteristic, the installation comprises at leasttwo molds and thus the same number of piston-cylinder assemblies; thefirst axle is shared by each of the assemblies and the second axles arearranged on the rotating structure at different positions equidistantfrom the pivot.

Each piston thus carries out the same movement as the other ones with aphase shift.

In these cases, the second axles are preferably spread angularlyregularly on the rotating structure.

The cycle is thus regular.

Others characteristics and advantages of the invention will become clearwhen reading the description of the attached figures below, in which:

FIGS. 1a and 1 b show in schematic form a compression chamber/moldassembly allowing implementation of the invention;

FIGS. 2a through 2 d show an improved version of a compressionchamber/mold assembly;

FIGS. 3a and 3 b show in schematic form the principle of an installationcomprising several assemblies corresponding either to that of FIGS. 1athrough 1 b or to that of FIGS. 2a through 2 d.

FIG. 1a shows a first possible form of construction of a device forimplementation of the invention when the chamber is at its maximumvolume.

The device comprises a cylinder 1 and a piston 2 defining a compressionchamber 3. The means of actuating the piston relative to the cylinderare not illustrated since this is a skeleton diagram.

The chamber 3 is connected by a fluid connection 4 to a low-pressurefluid source 5; in the example it is a source supplying industrial airat 7 bars.

A check valve 6 is interposed between the source 5 and the chamber 4.This valve makes it possible to bring the low-pressure into the chamber3 only when the residual pressure is lower than that of the source 5 (7bars in this instance). A valve is preferably as close to the chamber aspossible to limit dead volumes.

The chamber is also connected to a blow mold 7 by a fluid connection 8.

In the example, a blank 9 is present in the mold. In this instance, theblank 9 is a preform previously obtained by injection, then brought to ablowing temperature before insertion into the mold 7. It could be aparison obtained by extrusion, however.

Basically familiar means 10 such as a nozzle in the case of a preform ora hollow needle in the case of a parison are provided to connect thefluid connection 8 and the inside of the blank 9. These means, availableto the expert, as well as the means of positioning the blank in themold, and the means of closing the mold will not be described in greaterdetail.

A valve 11 is placed in the fluid circuit 8 and makes it possible toconnection the chamber 3 and the blank 9 when the piston positionattains a predetermined position.

For this purpose, a detector 12 of the piston position is provided thatis connected to an interface 13 controlling the valve 11 opening andclosing.

The detector 12 may be of any appropriate kind (optic, mechanical,electronic). It may be connected directly to the piston/cylinderassembly in order to directly determine the piston's position relativeto the cylinder; as an alternative, when this assembly is integratedinto a machine with a cyclic operation, the detector may be arranged todetermine at what stage of its cycle the machine is, so that theinterface 13 can give opening or closing orders at the appropriatetimes.

One and the same detector can thus control several valves by way of anappropriate interface.

In the example, the detector 12 is of the optic type. It detects thepassing of a mark 14 borne by the piston.

In addition, a safety valve 15 is preferably provided to avoid anydeterioration of the device in the event of accidental overpressure.This valve is calibrated at a few bars above the desired blowingpressure. It may be calibrated at 40 bars, for example, for an intendedblowing between 35 and 38 bars.

FIG. 1b illustrates the same device while the hollow body is in theprocess of being blown. The piston 2 is brought close to the end of thecylinder 1, and the valve 11 is opened. More specifically, the openingof the valve 11 occurred while the detector 12 had been activated by themark's 14 passing; the piston was then in the position shown by thebroken line 16.

Operation is as follows: when the piston is in the position of FIG. 1a,the initial pressure is established in the chamber (in this instance 7bars and the parallel valve is closed); the piston 2 is moved relativeto the cylinder 1 to reduce its volume, with the valve 11 always beingclosed: as a result, chamber pressure rises.

When the mark 14 passes in front of the detector 12 or, more generallyspeaking, when the interface 13 is activated, the valve 11 is opened:the hollow body starts its blowing. The piston then continues to move inorder to reduce the volume, continuing the compression and thus theblowing of the hollow body.

However, the structure that has just been described requires at eachblowing an initial pressurization of the chamber 3 by means of thelow-pressure circuit.

Using the examples provided in the preamble, that would require 1.5liters of air (or fluid) at 7 bars to blow a 300-cc container at 35bars, i.e., 10.5 liters of air at ambient pressure for each container ofthis volume.

This is why, as illustrated in these FIGS. 1a and 1 b, an additionalfluid connection 17 is preferably provided in order to return thehigh-pressure fluid contained in the hollow body to the chamber 3 afterblowing and at the time of degassing.

This connection joins the chamber 3 with the section of the connection 8between the valve 11 and the mold 7. A check valve 18 is arranged inthis connection to prevent the fluid contained in the chamber 3 fromdirectly entering the mold 7 at the time of compression or blowing.

As illustrated in these figures, the connection 17 preferably does notend at the bottom of the cylinder but on the cylinder wall in an areabetween the two end positions of the piston. Thus, after blowing, whenthe piston 2 is moved so as to increase the chamber volume and thenreturn to the initial position, the high-pressure air contained in thehollow body is not immediately reintroduced into the chamber. This makesit possible to keep the hollow body pressurized in the mold for a periodof time, for example to promote its definitive forming.

Then, when the piston 2 is moved so as to increase chamber volume, thepressurized fluid contained in the hollow body returns to the chamberwhen the connection 17 is no longer blocked by the piston.

However, since the device's total volume has increased during blowing, asimple return of the fluid is not sufficient to obtain the desiredinitial pressure: fluid is thus supplied automatically from the source5.

In addition, to promote the sending of the fluid from the hollow body tothe chamber and reduce the supply of low-pressure fluid as much aspossible, the connection 4 ends in the cylinder wall between the outletof the connection 17 and the piston's 2 distal position relative to thecylinder bottom, that is, the position in which the chamber volume is atits maximum.

Thus, after blowing and when the chamber volume increases, the aircontained in the hollow body is inserted into the chamber before that ofthe low-pressure source 5 that only provides the necessary supply.

The device of FIGS. 1a and 1 b thus makes it possible to blow hollowbodies practically automatically. However, it requires the use of avalve which, without being stressed in the same way as with the devicesof the prior art since it undergoes cyclic pressure changes, is stillnot continuously subjected to high pressure.

However, the device illustrated in FIGS. 2a through 2 d which no longerrequires a valve nor a detector and allows automatic blowing ispreferred to the preceding one.

The device comprises a piston 19 sliding in a cylinder 20 to define acompression chamber 21. A safety valve 15 is provided as in the cases ofFIGS. 1a and 1 b.

The piston cannot move between a position where chamber volume is at itsmaximum (FIG. 2a) and a position in which this volume is at a minimum(FIG. 2c). A fluid pipe or connection 22 ends with a first tip in thecylinder 20, through its wall, in an area opposite the piston 19 at anytime during its stroke. This pipe 22 is connected by its second tip withthe basically known blowing means 10 connected to the mold 7.

A check valve 23 is arranged on the pipe 22 to prevent fluid (air) fromreturning to the cylinder, as will be explained later.

An opening 24 is arranged in the mass of the piston. It ends on the onehand in the compression chamber 21 and on the other hand in acountersinking 25 arranged at the periphery of the piston.

The position of the countersinking is such that when the piston arrivesin the defined position to initiate blowing, the countersinking arrivesopposite the pipe 22 in such a way that the pipe is no longer blockedand a connection is then ensured between the chamber 21 and the pipe 22through the opening 24 and the countersinking 25 arranged in the piston.

In addition, the dimensions of the countersinking are such that whilethe piston continues its stroke to reduce chamber volume, the compressedfluid continues to make its way toward the mold 7 through the opening24, the countersinking 25 and the pipe 22.

Thus, FIG. 2b illustrates the moment when the piston 19 in the processof compression has just gone beyond the position to initiate blowing:fluid circulation may set in between the chamber 21 and the mold 7, andmore specifically between the chamber and the blank 9 arranged in themold.

FIG. 2c thus illustrates the piston position at the end of blowing:chamber 21 volume is at a minimum but the mold 7 is always connectedwith the chamber by way of the opening 24, the countersinking 25 and thepipe 22.

The function of the check valve 23 located on the pipe 22 is to preventthe pressurized fluid contained in the hollow body when blowing isfinished from returning to the chamber 21 when the piston initiates itsreturn movement to its initial position, while the countersinking 25 isstill opposite the opening 22. The high pressure is thus maintained atthe end of blowing in the hollow body, allowing it to stabilize.

Another pipe 26 connects the chamber 21 and a low-pressure fluid source5 in order to initialize pressure in the chamber when the pistonposition is such that chamber volume is at a maximum (or near itsmaximum).

A check valve 27 is provided in the pipe 26 to prevent fluid fromescaping toward the source 5 when the piston 19 compresses the fluid inthe chamber.

The structure that has just been described makes it possible to achieveautomatic blowing of the hollow body. However, it requires pressureinitialization to be completely carried out with the external source 5.

For this reason, to avoid excessive consumption on the external source5, in a preferred form of construction at least part of the pressurizedfluid contained in the hollow body at the end of blowing is evacuated tothe chamber.

For this purpose, a pipe 28 is provided between the chamber and themold. A check valve 29 is arranged on this pipe in such a way that itblocks any transfer of fluid from the chamber 21 to the mold 7 throughthe pipe 28. This valve allows only a transfer from the hollow body tothe chamber when residual pressure in the hollow body is greater thanthat of the chamber.

To make it possible to maintain pressure in the hollow body at the endof blowing, for example to help stabilize it, the pipe 28 does not endin the cylinder bottom but through its wall, in a position that isblocked by the piston during part of its return movement to maximumvolume. The pipe 28 thus only allows the transfer of fluid from thehollow body to the chamber when the piston starts releasing the pp.Thus, in FIG. 2d the pipe 28 is completely released by the piston inmotion in order to increase chamber volume: the fluid in the hollow bodycan make its way toward the chamber (as long as pressure in the hollowbody is greater than that of the chamber).

As in the cases of FIGS. 1a and 1 b, the device does not allow acomplete initialization of the chamber after blowing of a hollow bodyfrom the pressure contained in this hollow body, particularly because ofthe volume change due to the blowing.

However, as in the preceding case and preferably in order to promotereuse of the fluid contained in the hollow body, the pipe 26 connectedto the low-pressure source 5 is arranged in such a way that the maximumfluid contained in the hollow body is transferred to the chamber beforethe low-pressure fluid is sent to the chamber.

To do this, for example, as illustrated by FIGS. 2a through 2 d, thispipe 26 ends in an area of the cylinder wall farther from the cylinderbottom than the pipe 28 for transferring the fluid from the hollow bodyto the chamber. Thus, the low-pressure fluid can only be transferredfrom the source 5 to the chamber 21 when the piston is not opposite thepipe 26 and when pressure in the chamber is lower than that of thesource.

In FIG. 2a, the piston is in maximum volume position; the pipe 26 isunblocked: initialization can take place. It should be noted that if ahollow body was blown beforehand, part of the fluid it contained hasbeen reinserted into the chamber 21 since the pipe 28 is open in thechamber.

In FIG. 2b, the piston 19 has compressed the fluid in the chamber 21 andthe pipe 22 arrives opposite the countersinking: blowing begins.

In FIG. 2c, the piston is at the end of its stroke (minimum volume);compression is completed but the fluid contained in the hollow bodycannot return to the chamber due to the action of the check valve 23 andthe blocking of the pipe 28 by the piston.

In FIG. 2d, the pipe 28 is released following the piston's 19 movementto the maximum volume position: a transfer can take place from thehollow body to the chamber 21.

FIGS. 3a and 3 b illustrate the skeleton diagram of an installationimplementing the invention in very advantageous manner.

In the example, the installation comprises four piston 30 a, 30 b, 30 c,30 d—cylinder 31 a, 31 b, 31 c, 31 d assemblies each connected to itsrespective mold 32 a, 32 b, 32 c, 32 d.

A rotating structure 33 such as a carrousel is caused to turn around apivot 31 borne by the installation's chassis 35.

A motor 36, preferably electric, draws the carrousel by a belt 37.

In the preferred form of construction illustrated by these figures, eachpiston is attached to the first end of a respective stem 38 a, 38 b, 38c, 38 d, the other end of which is connected to a shared axle 39 borneby the chassis 35, parallel to the pivot's axis and at a distance fromit corresponding to half of the piston's stroke.

As for the cylinders, they are hinged on a respective axle 40 a, 40 b,40 c, 40 d borne by the rotating structure 33 toward the periphery ofthe structure. Each of the axles is equidistant from the pivot 34.

Alternately, it is conceivable to hinge the cylinders on the shared axle39, and the pistons by their stems on the respective axles 40 a, 40 b,40 c, 40 d. The encumbrance would be difficult to control, however.

As illustrated by these FIGS. 3a and 3 b, the axles 40 a, 40 b, 40 c and40 d are preferably angularly spread in regular manner over the rotatingstructure.

Due to the eccentricity between the pivot 34 and the shared axle 39, arotation of the structure causes each piston to move in its respectivecylinder. In addition, due to this eccentricity, the stems 38 a, 38 b,38 c and 38 d as well as the cylinders oscillate in the horizontal planerelative to the shared axle 39 and the respective axles 40 a, 40 b, 40 cand 40 d, respectively.

A low-pressure source 5 to initialize pressure in the cylinders isarranged outside the rotating structure and is connected to it by a pipe41 ending on a rotating, low-pressure attachment 42, preferably alignedalong the pivot's 34 axis.

Pipes 43 a, 43 b, 43 c, 43 d start from the rotating attachment towardthe cylinders.

In order not to overload these FIGS. 3a and 3 b, details of theconnections between each mold and the related cylinder-piston assemblywere not illustrated, all the more since the skeleton diagramillustrated by FIGS. 3a and 3 b applies indifferently to the firstvariant (FIGS. 1a and 1 b) and to the second one (FIGS. 2a through 2 d).

In the same way, details of the devices for opening/closing the molds,inserting the blanks and removal of the hollow bodies were not shown.

Assuming the structure is rotating in the direction of the arrow 44,operation is established as follows.

The chamber defined by the piston 30 a and the cylinder 31 a is at itsmaximum volume. This corresponds to the position of the FIG. 2a.

The corresponding mold 32 a is shown open because when a piston is inthis position, it is possible to unload a completed hollow body and loada blank.

In fact, unloading may start slightly before the piston has attainedthis position, i.e., more specifically when the hollow body is degassed.

As for the closing of the mold after a blank is loaded, this must beeffective before the piston has attained the defined position as ofwhich blowing can begin.

The piston 30 b is in the process of compressing the correspondingchamber, without having attained the blowing position.

The piston 30 c is in its end position. Blowing is completed in thisassembly. For the reader's information, this corresponds to the positionof the FIG. 2c.

The piston 30 d is in motion, increasing the corresponding chamber'svolume. This corresponds approximately to the position of the FIG. 2d(transfer of the fluid from the hollow body to the chamber).

In FIG. 3b the piston 30 a has begun its compression stroke; the piston30 b has attained the position where blowing can begin; the piston 30 cis an intermediate position between the position of FIGS. 2c and 2 d(hollow body stabilization); the piston 30 d is in a position wherepressures are balanced between the chamber and the hollow body. The moldcan thus be opened to remove the hollow body; at the same time, fluidmay be supplied from low-pressure fluid source 5.

As the preceding shows, the invention is applicable to injection-blowmolding or extrusion-blowing installations. It offers a fundamentaladvantage in the case of injection-blow molding installations since itmakes it possible to dispense with the high-pressure rotatingattachments and the problems inherent in compressors.

It has a particular advantage for blowing small items, that is, forvolumes on the order of one liter or less: beyond that, if one wants toremain dependent on industrially available low pressures, thecompression chamber volumes must be increased in particular and thisquickly leads to an unrealistic encumbrance.

Of course, the invention is not limited to the forms of constructiondescribed and specifically claimed; it covers all equivalents thereofwithin the expert's reach.

What is claimed is:
 1. Method for manufacturing a hollow body by blowinga blank into a finishing mold with the help of a blowing fluid,comprising connecting to each mold a separate compression chamber formedby a cylinder-piston assembly; establishing an initial fluid pressure inthe chamber when it is at its maximum volume; reducing the chambervolume to compress the fluid while keeping the chamber and the inside ofthe blank isolated; connecting the chamber and the blank when the pistonreaches a predetermined position in order to initiate blowing of theblank by retention of the compressed fluid, and continuing to reducechamber volume to a minimum while maintaining the connection with theblank and ending the blowing by compressing the fluid volume remainingin the chamber and transferring to the blank.
 2. Method according toclaim 1, characterized in that the initial pressure is established inthe chamber at least in part with the help of an external source oflow-pressure fluid connected to the chamber by a low-pressure circuit.3. Method according to claim 2, characterized in that the low-pressuresource provides a pressure ranging from 1 to 15 bars.
 4. Methodaccording to claim 3, characterized in that the low-pressure sourceprovides a pressure of 7 bars.
 5. Method according to claim 2, furtherproviding isolating means between the chamber and the low-pressure fluidcircuit in order to prevent the fluid contained in the chamber frombeing reinjected into the low-pressure circuit.
 6. Method according toclaim 1, characterized in that the initial pressure is established inthe chamber at least in part by returning the high-pressure fluidcontained in the body to the chamber when it is degassed.
 7. Methodaccording to claim 1, characterized in that the low-pressure sourcebrings fluid in addition to the degassing in order to obtain the initialpressure when a hollow body was blown in the mold during the precedingcycle.
 8. Method according to claim 1, characterized in that it consistsof keeping the pressurized fluid contained in the hollow body for adefined period after it has been blown.
 9. Device for implementing themethod according to claim 1, characterized in that it comprises,connected to a mold, a compression chamber constituted by a cylinder inwhich are arranged a piston; a fluid circuit connecting the chamber withmeans to establish an initial pressure in the chamber when it is at itsmaximum volume while being isolated from the inside of the blank; meansto connect the compression chamber with the inside of a blank placed inthe mold, when the piston position attains a predetermined position, andin that the length of the cylinder is such that after connecting thechamber with the blank, the stroke of the piston can continue andcomplete the blowing by transferring the fluid volume remaining in thechamber to the blank.
 10. Device according to claim 9, characterized inthat the means to connect the compression chamber with the blank areconstituted by a fluid connection ending on the one hand at the bottomof the chamber and on the other hand in the blank, and in that a valve,the opening and closing of which are controlled by piston positiondetection means connected to a control interface, is arranged in thisconnection.
 11. Device according to claim 9, characterized in that themeans for connecting the compression chamber and the blank comprise: onthe one hand, a pipe with a first tip that ends in the cylinder throughits wall, in an area opposite the piston at any time during its stroke,and with a second tip connecting the blowing means connected to themold; on the other hand, an opening arranged in the piston, with one endending in the compression chamber and a second end ending in acountersinking arranged at the periphery of the piston, and in that theposition of the countersinking is such that when the piston arrives inthe predetermined position to initiate blowing, the countersinkingarrives opposite the first end of the pipe, and in that the dimensionsof the countersinking are such that when the piston continues its stroketo complete the blowing, the countersinking is still opposite the pipe.12. Device according to claim 11, characterized in that a check valve isarranged on the pipe to prevent fluid from returning from the hollowbody to the chamber when the piston has completed its stroke andinitiates its return movement to its initial position, whereas thecountersinking is always opposite the opening.
 13. Device according toclaim 9, characterized in that the chamber is connected to a source oflow-pressure fluid by a connection in which check means are provided toprevent fluid from being returned to the source after compressionbegins.
 14. Device according to claim 13, characterized in that theconnection ends in the chamber opposite the wall of the cylinder in azone comprised between the two end positions of the piston in such a waythat this connection is closed by the piston during part of its stroke.15. Device according to claim 9, characterized in that the chamber isconnected with the mold by a connection provided with check means inorder, during compression, to prevent the fluid from being transferreddirectly to the mold while sufficient pressure has not been attained,and to allow the pressurized fluid contained in the hollow body toreturn to the chamber after blowing.
 16. Device according to claim 15,characterized in that the connection ends in the chamber opposite thewall of the cylinder, in a zone comprised between the two end positionsof the piston in such a way that this connection is closed by the pistonduring part of its stroke.
 17. Device according to claim 14,characterized in that the connection between the chamber and thelow-pressure source ends in a zone of the cylinder wall farther from thecylinder than the connection between the chamber and the mold. 18.Device according to claim 9, characterized in that it comprises a safetyvalve arranged at the bottom of the chamber to prevent overpressure. 19.Installation comprising at least one device according to claim 9,characterized in that it additionally comprises a chassis, a structurerotating around a pivot borne by the chassis and at least one moldattached to this structure and connected to its respective device; andalso in that a first end of each cylinder-piston assembly is connectedto a first respective axle borne by the chassis parallel to the pivot'saxis and at a distance from it that defines the desired piston stroke;and in that a second end of each cylinder-piston assembly is connectedto a respective second axle parallel to the first and borne by therotating structure toward a peripheral zone thereof.
 20. Installationaccording to claim 19, characterized by a piston comprising a controlstem which is connected to the first axle and the cylinder is connectedto the second axle.
 21. Installation according to claim 19,characterized in that it comprises at least two devices, and in that thefirst axle is shared by each of the devices and the second axles arearranged on the rotating structure at equal distance from the pivot. 22.Installation according to claim 21, characterized in that these secondaxles are angularly spread regularly on the rotating structure. 23.Installation according to claim 19, characterized in that it isconnected to an external low-pressure source and a low-pressure rotatingconnection is arranged in the axis of the piston.